Hydrocracking processes are well known and are used in a large number of petroleum refineries. Such processes are used with a variety of feeds ranging from naphthas to very heavy crude oil residual fractions. In general, a hydrocracking process splits the molecules of the feed into smaller (lighter) molecules having higher average volatility and economic value. At the same time, a hydrocracking process normally improves the quality of the material being processed by increasing the hydrogen-to-carbon ratio of the materials, and by removing sulfur and nitrogen. The significant economic utility of the hydrocracking process has resulted in a large amount of developmental effort being devoted to the improvement of the process and to the development of better catalysts for use in the process.
A hydrocracking unit consists of the two principal sections for reaction and separation, the configuration and types of which vary. There are a number of known process configurations, including once-through, or series flow, two-stage once-through, two-stage with recycle, single stage and mild hydrocracking. Parameters such as feedstock quality, product specification, processing objectives and catalysts determine the configuration of the reaction section.
In the once-through configuration, two reactors are used. The feedstock is refined over hydrotreating catalysts in the first reactor and the effluents are sent to the second reactor containing amorphous or zeolite-based cracking catalyst(s). In the two-stage configuration, the feedstock is refined over hydrotreating catalysts in the first reactor and the effluents are sent to a fractionator column to separate the H2S, NH3, light gases (C1-C4), naphtha and diesel products boiling in the range nominal 36-370° C. Hydrocarbons boiling at a temperature above 370° C. are then recycled to the first stage reactor or the second reactor.
In both configurations, hydrocracking unit effluents are sent to a distillation column to fractionate the naphtha, jet/kerosene, diesel and unconverted products boiling in the nominal ranges 36-180° C., 180-240° C., 240-370° C. and above 370° C., respectively. The hydrocracking products jet/kerosene (i.e., smoke point>25 mm) and diesel products (i.e., cetane number>52) are of high quality and well above worldwide transportation fuel specifications.
One of the advantages of the two-stage configuration is that it maximizes the mid-distillate yields. The converted products from the first stage are fractionated and not subjected to further cracking in the second reactor, resulting in a high mid-distillate yield.
A conventional two-stage hydrocracking unit of the prior art with recycle is schematically illustrated in FIG. 1. In the configuration shown, the feedstock 11 is hydrocracked in the first reactor 10 over hydrotreating catalysts, usually amorphous-based catalysts containing Ni, Mo or Ni, W or Co, Mo metals as the active phase. The first reactor effluent stream 12 is then passed to fractionator 20 and the light fractions 21 containing H2S, NH3, C1-C4 gases, naphtha and diesel fractions boiling up to a nominal temperature of 370° C. are separated. The hydrocarbon fraction 22, boiling above 370° C. are sent to the second reactor 30 containing amorphous and/or zeolitic-based catalyst(s) containing Ni, Mo or Ni, W metals as the active phase. The second reactor effluents stream 31 is recycled to the fractionator 20 for separation of the lighter cracked components.
The configuration of the separation section depends upon the composition of the reactor effluent. The reactor effluents are sent either to a hot separator or a cold separator. In the latter case, the reactor effluents, after passing the feed/effluent exchangers, are sent to a high pressure cold separator. A portion of the unconverted recycle stream is withdrawn from the fractionators bottoms as bleed stream 24. The gases are then recycled back to the reactor after being compressed and the bottoms are sent to a low pressure low temperature separator for further separation.
In the hot scheme, the reactor effluents are passed through the exchangers and are sent to a high pressure hot separator, from which the gases are recycled to the reactor. The bottoms are sent to a high pressure cold separator and to a low pressure low temperature separator for further separation.
Hydrocracking units utilizing a cold separator are usually designed for processing lighter feedstocks ranging from naphtha to diesel. Hydrocracking units utilizing a hot separator are designed for heavier feedstocks, vacuum gas oil and heavier components. There are advantages and disadvantages to both schemes. The surface area of the feed/effluent heat exchangers is reduced significantly in the scheme utilizing a hot separator. It is not necessary to cool all the effluents to 40° C. and preheat the stripper as in the cold scheme. Because of the heat efficiency, this scheme also results in a heat gain for feed preheating, which is about 30-40% of the cold scheme furnace requirement. A disadvantage of the hot scheme is that the recycle gas is generally less pure than that obtained in the cold scheme, which results in a higher reactor inlet pressure. The hydrogen consumption is also slightly higher with the hot scheme due to a higher hydrogen solubility.
Single stage once-through hydrocracking is a milder form of conventional hydrocracking. Operating conditions for mild hydrocracking are more severe than the hydrotreating process and less severe than the conventional high pressure hydrocracking process. This process is a more cost-effective hydrocracking process, but results in reduced product yields and quality. Mild hydrocracking processes produce less mid-distillate products of relatively lower quality compared to conventional hydrocracking process. Single or multiple catalysts systems can be used and their selection is based upon the feedstock processed and product specifications. Both hot and cold processing schemes can be used for mild hydrocracking, depending upon the process requirements. Single-stage hydrocracking uses the simplest configuration and these units are designed to maximize mid-distillate yield using a single or dual catalyst system. Dual catalyst systems are used in a stacked-bed configuration or in two series reactors.
Single-stage hydrocracking units can operate in a once-through mode or in recycle mode with recycling of the unconverted feed to the reactor. Hydrotreating reactions take place in the first reactor, which is loaded with an amorphous-based catalyst. Hydrocracking reactions take place in the second reactor over amorphous-based catalysts or zeolite-based catalysts. In the series-flow configuration, hydrotreated products are sent to the second reactor. In the recycle-to-extinction mode of operation, the reactor effluents from the first stage together with the second stage effluents are sent to the fractionators for separation, and the unconverted bottoms, free of H2S and NH3, are sent to the second stage. There are also variations of the two-stage configuration.
It is known in the prior art to use steam stripping to separate light components such as C1-C4 gases, and H2S and NH3. U.S. Pat. No. 6,042,716 discloses a process in which gas oil and hydrogen are reacted in the presence of a catalyst for deep desulfurization and deep denitrogenation. The effluent is steam stripped to separate the gas phase, and the liquid phase is dearomatized by reaction with hydrogen in the presence of a catalyst. In the examples given, the gas oil boils in the range of 184-394° C. and steam stripping is used to separate the gas phase from the liquid phase. Steam stripping is commonly used in refining operations to strip the hydrocarbon gases methane, ethane, propane and butanes and heteroatom-containing gases such as H2S and NH3.
In U.S. Pat. No. 5,164,070, steam is used to remove light gases and naphtha. However, the cut point is naphtha, the end boiling point of which is 180° C. In the process described, steam is preferably charged to the bottom of the stripping column through line 7 to effect stripping of the lighter hydrocarbons and more volatile materials from the entering liquids. Alternatively, a reboiler may be placed at the bottom of the stripping column to effect or aid in achieving the desired degree of stripping. The stripping column is intended to remove a large majority of naphtha boiling hydrocarbons from the entering liquid streams and to also remove essentially all lower boiling hydrocarbons. The remaining heavier hydrocarbons are discharged through line 8 as the net bottoms stream of the stripping column.
U.S. Pat. No. 5,447,621 discloses a mid-distillate upgrading process where steam is used to remove the volatile components but not the heavy fractions like diesel, which is the feedstock in this patent.
The processes disclosed in U.S. Pat. No. 5,453,177 and U.S. Pat. No. 6,436,279 utilize steam stripping to remove light end components.
U.S. Pat. No. 7,128,828 discloses a process which removes low boiling, non-waxy distillate hydrocarbons overhead using a vacuum steam stripper.
U.S. Pat. No. 7,279,090, steam stripping is used to separate the hydrocarbon fractions boiling in the range of 36-523° C. in a process that integrates solvent deasphalting and ebullated-bed residue conversion of vacuum residue feedstock boiling at 523° C., and higher and steam stripping is used to separate the residue from the other fractions boiling at 523° C. and below.
A number of references disclose the use of multiple hydrocracking zones within an overall hydrocracking unit. The terminology “hydrocracking zones” is employed herein as hydrocracking units often contain several individual reactors. A hydrocracking zone may contain two or more reactors. For instance, U.S. Pat. No. 3,240,694 illustrates a hydrocracking process in which a feed stream is fed into a fractionation column and divided into a light fraction and a heavy fraction. The light fraction passes through a hydrotreating zone and then into a first hydrocracking zone. The heavy fraction is passed into a second, separate hydrocracking zone, with the effluent of this hydrocracking zone being fractionated in a separate fractionation zone to yield a light product fraction, an intermediate fraction which is passed to the first hydrocracking zone and a bottoms fraction which is recycled to the second hydrocracking zone.
U.S. Pat. No. 4,950,384 entitled “Process for the hydrocracking of a hydrocarbonaceous feedstock” separates the first stage reactor effluent using a flash vessel. A hydrocarbonaceous feedstock is hydrocracked by contacting the feedstock in a first reaction stage at elevated temperature and pressure in the presence of hydrogen with a first hydrocracking catalyst to obtain a first effluent, separating from the first effluent a gaseous phase and a liquid phase at substantially the same temperature and pressure as prevailing in the first reaction stage, contacting the liquid phase of the first effluent in a second reaction stage at elevated temperature and pressure in the presence of hydrogen and a second hydrocracking catalyst to obtain a second effluent, obtaining at least one distillate fraction and a residual fraction from the combination of the gaseous phase and the second effluent by fractionation, and recycling at least a part of the residual fraction to a reaction stage.
U.S. Pat. No. 6,270,654 describes a catalytic hydrogenation process utilizing multi-stage ebullated bed reactors with interstage separation by flashing between the series of ebullated bed reactors. This process is carried out only on residual feedstocks boiling above 520° C.
U.S. Pat. No. 6,454,932 describes multiple-stage ebullating bed hydrocracking with interstage stripping and separating that employs a separation step, and stripping with hydrogen between the ebullated bed reactors. The process is carried out on feedstocks boiling at 650° C. and above, and is used on both vacuum distillates and residues.
U.S. Pat. No. 6,620,311 discloses a process for converting petroleum fractions that includes an ebullated bed hydroconversion step, a separation step, a hydrodesulfurization step, and a cracking step that utilizes a steam stripper.
U.S. Pat. No. 4,828,676 and U.S. Pat. No. 4,828,675 disclose a process in which a sulfur-containing feed is hydrogenated, stripped, and reacted with hydrogen in a second stage. Steam stripping is used to remove H2S (but not naphtha and diesel products) as shown in—col. 10, 1. 11; col. 11, 1. 7-10; col. 25, 1. 18-22.
Gupta U.S. Pat. No. 6,632,350 and U.S. Pat. No. 6,632,622 disclose a two stage vessel with stripping of first stage effluents in the same vessel. Gupta U.S. Pat. Nos. 6,103,104 and 5,705,052 disclose a two stage vessel with stripping of first stage effluents in a separate stripper vessel. The processes disclosed in the Gupta patents also remove dissolved gas in liquid with steam stripping.
U.S. Pat. No. 7,279,090 uses steam stripping to separate naphtha, diesel and VGO fractions boiling in the range 36-523° C. However, this patent claims an integrated process processing vacuum residue feedstock boiling at 523° C. and higher.